A more realistic perspective is that pacifism (even "realistic") is a form of deontological cowardice that abdicates the moral burden of making difficult tradeoffs to more mature actors... [who] did so without running away or denying these moral dilemmas but facing them head-on, accepting the karmic burden of consequentialist heroism.

If the tradeoff is difficult then it is difficult either way. Hiroshima was never the hiring pitch, no scientist at Los Alamos in 1944 thought they were averting an amphibious invasion, and none of them thought they were engineering some trembling hand perfect equilibrium at the end of human history - but for the sake of argument let us suppose otherwise. If choosing to help develop nuclear weapons to prevent a conventional invasion of Japan is accepting a "karmic burden" in the face of a "dilemma", then refusing to help develop them even at the risk of a realized Operation Downfall is not "cowardice". Virtue is not being right about what actually ended up occurring, it is having the sort of dispositions that tend to produce right belief and right action in general, without the benefit of hindsight.

Harry Truman was not special. Harry Truman was not some unique, world-historic genius. Harry Truman was just an ordinary person, with particular opinions and particular capabilities, sometimes good and sometimes bad, sometimes right and sometimes wrong. Some of his decisions were excellent; some were catastrophic fuckups that will haunt us all forever. He is not a hero for making some good choices any more than he is Hitler for making some bad ones, and it is the furthest thing from cowardice to refuse to substitute the judgement of the most powerful man on Earth for moral judgements of your own. If you want to blame Rotblat for being unwilling to suspend his judgement of state policy when it was good, you have to give him credit for not falling in line when it was bad, as so many others did. If you want to credit Teller and Von Neumann for the end of the last great power war, you have to blame them for their willingness to risk the final one. That, or you should abandon the language of personal virtue entirely, and just talk about decisions you agree with and decisions you do not.